The present invention relates to a light-emitting device made with gallium-nitride-group compound-semiconductor such as light-emitting diode, laser diode, etc.
Gallium-nitride-group compound-semiconductors have been increasingly used as the semiconductor material for the visible light-emitting devices and for use in the electronic devices of high operating temperature. The development has been significant in the field of blue and green light-emitting diodes.
In manufacturing the gallium-nitride-group compound-semiconductors devices, an insulating sapphire is generally used as the substrate for growing semiconductor film. Such devices are different from the light-emitting devices where semiconductor substrates other than gallium-nitride-group type substrates, such as, for example. GaAs or InGaP, are utilized. Specifically, those using an insulating substrate like the present sapphire have the n-side and p-side electrodes formed in one side of the substrate wherein the semiconductor film has been formed, because the electrodes can not be provided from the substrate.
Meanwhile, in the recent manufacture of light-emitting devices, including those using the sapphire substrate, the growing of gallium-nitride-group semiconductor thin film by a metal organic CVD method has become a main stream procedure. In the procedure, a substrate is placed in a reaction tube, and metal organic compound gas (tri-methyl-gallium [TMG], tri-methyl-aluminum [TMA], tri-methyl-indium [TMI], etc.) are supplied therein as the material gas for the Group III element, and ammonia, hydrazine, etc. as the material gas for the Group V element, while maintaining the substrate at a high temperature 900° C.–1100° C., to have an n-type layer, a light-emitting layer and a p-type layer grown on the substrate in a stacked structure. After the layers are grown and formed, the p-type layer and the light-emitting layer are partially etched off to have the n-type layer exposed, and then an n-side electrode and a p-side electrode are formed on the surface of exposed n-type layer and the p-type layer, respectively, for example by a deposition method.
Most of the recent light-emitting devices have the above described double-hetero-structure, fabricated by stacking the thin films of gallium-nitride-group compound-semiconductor on a sapphire substrate. FIG. 2 shows a cross sectional structure of a prior art light-emitting device of gallium-nitride-group compound-semiconductor.
In FIG. 2, a buffer layer 12, an n-type layer 13 of gallium-nitride (GaN), a light-emitting layer 14 of indium-gallium-nitride (InGaN), a p-type clad layer 15 of aluminum-gallium-nitride (AlGaN) and a p-type contact layer 16 of GaN are stacked on a sapphire substrate 11. A p-side electrode 17 is formed on the p-type contact layer 16, and an n-side electrode 18 is formed on an exposed surface of the n-type layer 13 provided by partially removing the following three layers, p-type contact layer 16, p-type clad layer 15 and light-emitting layer 14. The n-type electrode 18 is normally made with aluminum (Al), titanium (Ti), gold (Au), or such other metals. The light-emitting gallium-nitride-group compound-semiconductor devices of the above structure have been disclosed in, for example, Japanese Patent Publication No. 6-268259.
In the prior art light-emitting gallium-nitride-group compound-semiconductor devices of the above structure, the n-type layer 13 is formed of a gallium-nitride-group compound-semiconductor doped with n-type impurities such as silicon (Si), germanium (Ge). More specifically, during the growth of the n-type layer of gallium-nitride-group compound-semiconductor by said metal organic CVD method, silane, mono-methyl-silane, etc. are supplied, together with the material gas, as material gas for Si, or germane, mono-methyl-germane, etc. as material gas for Ge. The carrier concentration of n-type layer 13 may be controlled by adjusting the flow rate of the material gas for n-type impurities.
In the gallium-nitride-group compound-semiconductor, the n-type layer may also be formed by intentionally not doping the n-type impurities, because it exhibits the n-type property even without the n-type impurities being doped therein.
If in the light-emitting gallium-nitride-group compound-semiconductor devices the efficiency of light-emission is to be maintained high, the operating voltage needs to be lowered. In order to reduce the operating voltage, the series resistance in respective layers of compound-semiconductor stacked on the substrate 11 and the contact resistance with electrode have to be made low.
An effective means for reducing the series resistance of n-type layer 13 and the contact resistance with the n-side electrode 18 is to increase the doping quantity of n-type impurities during growth of n-type layer 13 by metal organic CVD. However, when doping quantity of the n-type impurities is increased, a strain can be generated in the grown n-type layer 13, which increases and readily leads to cracks at the n-type layer 13. If there are cracks in the n-type layer 13, an even emission of light may not be obtained over the entire surface, and the reliability of a light-emitting device may be degraded.
On the other hand, if priority is placed on suppression of cracks at n-type layer 13, the n-type layer 13 needs to be grown and formed in a reduced doping quantity of the n-type impurities. In this case, however, it becomes difficult to reduce the contact resistance with n-side electrode 18. If, in compensation of the above, the layer thickness of the n-type layer 13 is increased up-to about several μm (e.g. 16, 17 μm) in order to reduce the series resistance of a light-emitting device, cracks are easily induced like in the earlier described case. In addition, it needs a longer time for growing the crystal, which is an additional disadvantage in the manufacture thereof.
As described in the above, if in a light-emitting gallium-nitride-group compound-semiconductor device the doping quantity of n-type impurities is increased for lowering the operating voltage, or the layer thickness is increased, the occurrence of cracks may be unavoidable, which leads to a degraded light-emitting capability and a deteriorated manufacturing yield rate.
The problems expected to be solved by the present invention with a light-emitting gallium-nitride-group compound-semiconductor device using an insulating substrate are; first to reduce the operating voltage, and second to suppress the occurrence of cracks during growth for an improved manufacturing yield rate.